A lighting apparatus usually includes a laser as light source, for example, the radiation of which laser is directed onto a converter including phosphor, for example. The laser used generates blue light, for example, and the converter converts said blue light partly into yellow light. Such a lighting apparatus is referred to as LARP system (Laser Activated Remote Phosphor), for example. Specifically, it is possible to use such lighting apparatuses as module constructed in the automotive sector as motor vehicle headlight.
The optical safety monitoring of LARP modules, wherein for example a partially defective converter is intended to be detected, involves carrying out, for example, a measurement of the ratio of yellow light to blue light in the useful light. In this case, it is difficult to define a threshold value for the detection of a fault case. Causes of these difficulties are, inter alia, the fact that said ratio also changes if the power density at the converter or the temperature changes. This is because both parameters change the conversion efficiency of the converter and thus the yellow/blue ratio. Moreover, the surfaces in the optical path of the sensor signal may change in a wavelength-dependent manner during the lifetime. In addition, the efficiency of the converter may change over the lifetime.
The document EP 2 297 827 B1 discloses a method for operating a laser-based light source. At least part of the laser light is converted into converted light. A signal for the converted light which is related to the power of the emitted laser light is determined. A parameter safe for operation is determined on the basis of the laser output signal and the signal for the converted light. The parameter safe for operation is compared with at least one threshold value defined beforehand. Finally, the operation of the laser-based light source is controlled on the basis of the comparison of the parameter safe for operation with the threshold value defined beforehand.
Moreover, the document U.S. Pat. No. 8,400,011 B2 describes a lighting apparatus having improved safety visa vis the human eye. The lighting apparatus may be used for motor vehicles. A fluorescent plate is irradiated with the laser light and part of the laser light is reflected. This reflected part of the light is detected and used for the control of the laser.
Furthermore, the document DE 20 2015 01 682 U1 discloses a lighting device for a vehicle including a laser diode arrangement, a light wavelength conversion element for the wavelength conversion of the light generated by the laser diode arrangement, and also a safety apparatus for measuring a fault case of the light wavelength conversion element. The safety apparatus includes at least one signal transmitter and at least one signal receiver for transmitting and receiving measurement radiation. The at least one signal transmitter and the at least one signal receiver are arranged on different sides of the light wavelength conversion element, such that measurement radiation from at least one signal transmitter penetrates through the light wavelength conversion element.
In order to avoid the abovementioned problem of difficult fault detection when monitoring e.g. the yellow/blue ratio, often a beam trap is additionally used. In the fault case, the blue laser light is absorbed there. Said beam trap considerably reduces the useful luminous flux.